1. Field of the Invention
The present invention relates to a coordinated transmission method, coordinated transmission system, central station and radio base station for coordinating a plurality of radio base stations to transmit signals to a radio terminal.
2. Description of Related Art
In cellular mobile radio communication systems, to improve spatial frequency usage efficiency, it is required to minimize frequency reuse factor. For example, in the 3G cellular system using the CDMA (Code Division Multiple Access) system, one-cell frequency reuse is actualized (frequency reuse factor is “1”) where adjacent cells use radio signals of the same frequency band.
Meanwhile, on the downlink in the next-generation cellular system, OFDMA (Orthogonal Frequency Division Multiple Access) systems are strong. When one-cell frequency reuse is used in cellular systems using the OFDMA system, interference from adjacent cells and adjacent sectors becomes a significant cause of deterioration in characteristics. More specifically, since the SINR (Signal-to-Interference and Noise power Ratio) decreases as interfering power increases from adjacent cells and adjacent sectors, it is difficult to obtain the effects, particularly, when MIMO (Multiple-Input Multiple-Output) transmission is performed.
Accordingly, in the case that one-cell frequency reuse is used in the cellular system where MIMO transmission is performed in the OFDMA system, to obtain the effect of increasing throughput by MIMO transmission, it is necessary to avoid interference from adjacent cells and adjacent sectors.
Therefore, as the technique of avoiding interference in the above-mentioned case, attention is directed toward coordinated transmission where a plurality of radio base stations are coordinated to simultaneously transmit signals to one or more radio terminals. By this coordinated transmission, since it is possible to orthogonalize the space inside a coordinated cluster formed by a set of cells or sectors of the plurality of coordinated radio base stations, interference inside the coordinated cluster (i.e. interference among radio base station apparatuses that perform coordinated transmission) is avoided (for example, Non-patent Document 1 and Non-patent Document 2, Non-patent Document 1: A. Benjebbour, M. Shirakabe, Y. Ohwatari, J. Hagiwara, and T. Ohya, “Evaluation of user throughput for MU-MIMO coordinated wireless networks,” IEEE PIMRC 2008, pp. 1-5, September 2008, Non-patent Document 2: CMCC, “Downlink CoMP-MU-MIMO transmission schemes,” 3GPP RAN1 #56, R1-090922, February 2009).
More specifically, in coordinated transmission as shown in Non-patent Document 1, as shown in FIG. 1, a plurality of adjacent radio base stations 1 are connected to a central station 2, and the central station 2 coordinates the plurality of radio base stations 1. A coordinated cluster CL is formed by a set of cells C of the plurality of coordinated radio base stations 1. Inside the coordinated cluster CL is performed coordinated transmission by the plurality of radio base stations 1 by multiuser MIMO transmission. As a precoding method of the multiuser MIMO transmission, when a Block Diagonalization Zero-forcing (BD-ZF) method is used, since the space inside the coordinated cluster CL can be orthogonalized, it is possible to cancel interference inside the coordinated cluster, and user throughput is improved.
Meanwhile, in coordinated transmission as shown in Non-patent Document 2, as shown in FIG. 2, three adjacent radio base stations 1 are connected to a central station 2, and the central station 2 coordinates the radio base stations 1. A coordinated cluster CL is formed by three adjacent sectors of three coordinated radio base stations 1. Further, the three adjacent sectors forming the coordinated cluster CL are constructed so that beams from sector antennas from respective radio base stations 1 face one another. Inside the coordinated cluster CL is performed coordinated transmission by three radio base stations 1 by multiuser MIMO transmission. As a precoding method of the multiuser MIMO transmission, when the Block Diagonalization Zero-forcing (BD-ZF) method is used, since the space inside the coordinated cluster CL can be orthogonalized, it is possible to cancel interference inside the coordinated cluster, and user throughput and cell throughput is improved.
However, in the above-mentioned interference avoiding techniques using coordinated transmission by a plurality of radio base stations, although it is possible to avoid interference inside the coordinated cluster (i.e. interference among radio base stations that perform coordinated transmission), there is a problem that it is not possible to avoid interference from the outside of the coordinated cluster (i.e. interference from radio base stations that do not perform coordinated transmission).